Besides attempting answers in astrophysics [1]}, [2]}, [3]} and cosmology [4]}, [5]} in a manner
complementary to electromagnetic astronomy, GWs are unique probes of
fundamental physics. For more than a century, Albert Einstein's theory
of General Relativity (GR) has been our description of
gravitational interactions, having passed every experimental and
observational challenge, so far. However, for the first time, the
LIGO-Virgo GW observations have allowed us to probe GR in the
large-velocity, highly dynamical, strong-field regime of gravity,
a regime which is inaccessible with tests in the Solar
System [6]}, in binary pulsars [7]}, and
with observations around supermassive BHs at the center of
galaxies [8]}, [9]}, [10]}.
